1. Field of the Invention
The present invention relates generally to the field of optical communications and more particularly to the fabrication of passive and active devices used in dense wavelength division multiplexing (DWDM) applications.
2. Background Art
Optical communication has been an active area of development and is crucial to the enhancement of several key technological advancements, e.g., Internet and related new technologies. An important technology that enabled a higher data transmission rate is the dense wavelength division multiplexing (DWDM) technology. In many DWDM applications, phase and free spectra range (FSR) adjustable reflecting elements are key components in passive and active devices. As depicted in prior art FIG. 1, a frequently used FSR adjustable device consists of two transparent windows (12, 14) with partially reflective coatings (16, 18) and a precision spacer (10) made of piezo-electric material. The device is frequently referred to as an etalon if the reflectivities of both surfaces are not unity. When one of the surfaces has a reflectivity very close to unity, the device is commonly known as a Gires-Tournois (GT) mirror. The FSR of the device (in Hz) is related to the index of refraction n of the medium inside of the cavity, the thickness of the cavity spacer d, in accordance with FSR=c/(2nd). Here c is the speed of light. Since FSR is determined by optical thickness (nd) of the device, the FSR can be adjusted by changing the thickness of the spacer (i.e., by applying a predetermined voltage). The disadvantage of this type of mirror device is that the thickness of the device may be temperature sensitive and an active voltage is required to maintain a specific value of FSR. Another frequently used two-reflector device is illustrated in prior art FIG. 2. Similar to the device presented in FIG. 1, there are two transparent windows (22, 24) with partially reflective coatings (26, 28) and a precision spacer (20). In addition, there are two transparent precision plates (25, 27) which can be rotated to change their optical thickness. The orientation of 25 may be changed to adjust the FSR of the device whereas 27 may be rotated to change the phase of the incoming light through the device. Such phase and FSR adjustable reflecting devices have found wide usage in DWDM applications. Another prior art, polarization modifying reflecting device is illustrated in FIG. 3. Similar to the devices presented in FIG. 1 and in FIG. 2, there are two transparent windows (32, 34) with partially reflective coatings (36, 38) and a precision spacer (30). In addition there are two transparent precision wave plates (35, 37) which can be rotated to change their optical thickness. The orientation of 35 may be changed to adjust the FSR of the device whereas 37 may be rotated to change the phase of the incoming light through the device. A recently disclosed reflecting device is illustrated in prior art FIG. 4. Similar to the devices presented in FIG. 1 and FIG. 2, there are two transparent windows (42, 44) with partially reflective coatings (46, 48) and a precision spacer (40). In addition there are two transparent precision phase plates (45, 47) which can be rotated to change their optical thickness. The orientation of 45 may be changed to adjust the FSR of the device whereas 47 may be rotated to change the phase of the incoming light through the device. Both phase plates (45, 47) have half of the area covered with other transparent materials. The thickness of each additional half plate is predetermined to give 90 and 180 degrees of relative phase shifts, respectively, for particular wavelengths range required in specific applications. A common problem associated with reflecting devices illustrated in FIGS. 2, 3 and 4 is that the adjustable phase and/or wave plates are difficult to adjust and frequently cause reliability problems in stringent telecom applications.
The most relevant prior art patent appears to be U.S. Pat. No. 6,169,604 to Cao issued Jan. 2, 2001. The Cao patent discloses some of the aforementioned prior art concepts and is therefore incorporated herein by reference as relevant background material.
The present invention discloses nano tuner (NanoT) reflectors with hermetically sealed cavities and the ways to introduce controlled or adjustable optical medium into these cavities. The optical density of the medium in each of the cavities may be adjusted to yield desired phase and/or FSR values for specific applications. These NanoT reflectors consist of one or two cavities, an optical medium tuner, and phase and wave plates. The optical functions of these NanoT reflectors are identical to the corresponding prior art reflecting devices. Various embodiments are disclosed. Each provides at least one hermetically sealed optical cavity connected to a tuning device through a channel. The tuning device permits modification to the density of an optical medium contained in the cavity. In the preferred embodiments disclosed, the optical medium comprises one or more selected gases which determine the index of refraction in the cavity. The cavity is formed between two optical windows having facing surfaces coated by partially or wholly reflective coatings. The facing surfaces are separated by a selected distance by a precise spacer. In some embodiments, two cavities are employed and each may contain a wave plate or phase plate.